The body constantly expends energy, even at rest, to maintain life-sustaining functions like breathing and circulation. This resting energy use is known as the Basal Metabolic Rate (BMR). Total Energy Expenditure (TEE) includes BMR along with energy used for physical activity and food digestion. When the body is under stress from an infection, a common question arises: does the internal fight against illness increase the body’s energy demands?
Does Illness Increase Calorie Expenditure?
Yes, being sick generally increases your body’s caloric expenditure because fighting an infection requires a significant amount of energy. This rise in energy use is part of a complex physiological shift often called “sickness metabolism.” Your body temporarily prioritizes energy resources away from non-immediate functions to fuel the defense mechanisms against the invading pathogen.
The extent of this metabolic acceleration depends directly on the severity and type of the illness. A minor head cold, for instance, may cause only a negligible increase in energy burn. However, more severe systemic infections or trauma can trigger a pronounced “stress-induced hypermetabolic state.” In cases of severe illness, such as sepsis, resting energy expenditure can rise significantly, sometimes increasing the BMR by 20% or more.
The Role of Immune Response and Fever
The heightened caloric burn during sickness is primarily driven by two energy-intensive processes: the activation of the immune system and the generation of fever. The immune response is a massive mobilization of cellular resources, demanding a substantial energy supply. Producing new immune cells, such as T-cells and B-cells, and manufacturing specific antibodies consume significant amounts of adenosine triphosphate (ATP), the body’s energy currency.
Inflammatory mediators called cytokines signal the body to ramp up energy production to support these immune functions. This chemical signaling ensures that the immune system has the fuel it needs for the rapid multiplication and deployment of defense components. The resulting metabolic acceleration allows immune cells like neutrophils and lymphocytes to meet their heightened energetic demands while engaged in the fight.
Fever represents the most measurable cause of increased calorie expenditure while sick. When the hypothalamus raises the body’s temperature set point in response to infection, the body actively works to generate and maintain that higher temperature. This process, known as thermogenesis, often involves shivering and requires a constant energy investment.
Scientific estimates indicate that for every 0.5 degrees Celsius increase in core body temperature, the basal metabolic rate rises by approximately 7%. This means a sustained fever of even a few degrees can substantially increase daily caloric needs. This increase is due to the energy required to sustain the elevated temperature.
Nutritional Considerations While Sick
Given the hypermetabolic state induced by illness, maintaining adequate nutritional intake becomes a necessity to support recovery. Despite the body burning more calories, appetite is often suppressed due to general discomfort, nausea, or congestion. This combination creates an energy deficit, which forces the body to use stored reserves.
If adequate calories are not consumed, the body may begin breaking down muscle tissue, a process called catabolism, to provide the necessary energy and amino acids. During illness, a higher percentage of the body’s energy can be derived from muscle protein breakdown, meaning sufficient protein intake is necessary to preserve muscle mass. Lean proteins supply the essential amino acids that are the building blocks for new immune cells and antibodies.
Hydration is also a serious consideration, particularly when fever is present, as increased body temperature and sweating lead to greater fluid loss. Dehydration can further stress the body and interfere with its ability to regulate temperature and transport nutrients. Consuming fluids is critical to supporting all bodily functions, including the immune response.
Specific micronutrients play supporting roles in immune function. Key elements like Vitamin C and Zinc support various immune cell activities and the formation of antibodies. Ensuring sufficient intake of these vitamins and minerals provides the necessary co-factors for the immune system to operate at its increased capacity.